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Solar Joule Bracelet 

Make] Projects 

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build, hack, tweak, share, discover,- 

Solar Joule Bracelet 

Written By: Edwin Wise 




• BoltM) 



• Digital multimeter (1) 

from RadioShack 

from RadioShack 

i • 

Photodiodes (10+) 

Gauged wire cutter (1) 

such as manufacturer #BPW34 

from RadioShack 


Supercapacitor (1) 

Pliers (1) 

such as manufacturer 

from RadioShack. A second pair is 


handy for twistina the wire 


■ • 

Schottky signal diode (1) 

• Soldering iron (1) 

from RadioShack 

from RadioShack 


Common-mode choke (1) 

• Third-hand tool (1) 

such as manufacturer #CC2824E513R- 

from RadioShack 


• • 

Transistor (1) 
from RadioShack 


Resistor (1) 
from RadioShack 

i • 

Capacitor (1) 

(optional) such as manufacturer 


; • 


© Make Projects 

Pagel of 12 

Solar Joule Bracelet 

from RadioShack 

Fabric (1) 

or other bracelet material: for mounting 


My wife fell in love with Alice Planas and Hatti Linn's glowing bracelet project from CRAFT 
Volume 06 ("Solar Jewelry," page 123), so of course we had to make one. I built the circuit 
and she did the fabric, and it came out nicely, but I felt it could be brighter. So I decided to 
combine the solar jewelry idea with a Joule Thief circuit, which converts low voltages into 
short bursts of voltage high enough to light an LED. 

This project is the result: the Solar Joule, which combines Solar Jewelry with a Joule Thief. 
(Note that the joule is a standard measure of energy, and is pronounced WWe jewel.) 

Joule Thief Theory 

The Joule Thief circuit is a fairly simple way of converting very low voltages, like from dead 
batteries, into an oscillating voltage that's high enough to do something useful. There are 
many versions online (for example, see ). The heart of the circuit is a 
pair of inductor coils wound together into a transformer or choke. When current runs into one 
coil of a choke, it's resisted until it builds up a magnetic field, which then draws current 
through the other coil, going in the opposite direction. In the Joule Thief, one coil provides 
the kick of voltage that overcomes the LED's forward voltage requirement, and the other 
generates feedback that drives a transistor into oscillations. 

© Make Projects Page 2 of 1 2 

Solar Joule Bracelet 

Step 1 — How the feedback works. 

© Make Projects Page 3 of 1 2 

Solar Joule Bracelet 

• Here's the feedback diagram. 
When you first connect power to 
the circuit, the transistor is off. 
There is no magnetic field in the 
choke, and there's not enough 
power to turn on the LED. Some 
power leaks through the resistor 
into the transistor's base, turning it 
on a little bit. This lets a small 
amount of current run backward 
through inductor coil 1-2 of the 
choke, creating a small expanding 
magnetic field. As a result, current 
is forced through inductor 2>-4, 
which turns the transistor on even 
more. This positive feedback loop 
continues until the transistor is 
completely activated. 

• Once the transistor is done 
opening, the current through 1-2 
stops increasing, so the magnetic 
field stops expanding and the 
current through 4-3 equalizes. This 
causes the transistor to close a 
little bit, which initiates a feedback 
loop in the opposite direction. 
Current reverses through 4-3, 
which closes the transistor and 
draws current through 1-2. When 
the transistor shuts off, the 
inductor's magnetic field winds 
down and unloads a blob of charge 
at pin 2. Once this exceeds the 
LED's forward voltage, the LED 
lights up. The current is quickly 

© Make Projects 

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Solar Joule Bracelet 

The capacitor between the resistor 
and the choke provides a little 
"spring" to the feedback action, 
speeding it up and buffering some 
of the voltage changes across 
inductor 4-3. 

Step 2 — Build the solar battery. 

• The solar battery is a series of 
photodiodes bridged by a 
supercapacitor that stores the 
energy they collect. The 
photodiodes also act as the links in 
the chain bracelet, and I connected 
them with loops of wire to provide 
some spring and make it easy to 
sew them onto fabric. 

© Make Projects 

Page 5 of 1 2 

Solar Joule Bracelet 

Step 3 — Make the photodiode chain. 

m * ^*. 

I'ii* > ii^A 

• To make the loops, wind solid wire around a small bolt (image 1), cut it at every other turn, 
and re-bend the ends of each loop into small solderable hooks (image 2). 

• TIP: Use a second pair of pliers to hold the screw, and try cutting at every third turn 
if your loops are too small too work with comfortably. 

• Solder 10 (or more) photodiodes in series, + to - (image 3). The silver stripe on each 
diode's face indicates the + side. (If you want to go crazy, make 2 strips of photodiodes 
and connect them in parallel, side by side, + to + and - to -.) 


© Make Projects 

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Solar Joule Bracelet 

Step 4 — Add the supercap and Schottky diode. 

• Solder the Schottky diode to the + 
end of the series, with the diode's 
black stripe (the - end) pointing 
away from the PIN photodiodes. 

• NOTE: The Schottky 
ensures that power flows 
into the capacitor when the 
photodiodes are in bright light, but 
won't flow back out when they're 
dark. Any diode will work here, but 
a Schottky diode consumes less 
voltage in the forward direction, 
saving more for the LED. 

• Solder in the supercapacitor, with 
its + side connected at the signal 
diode and its - side (marked with 
an arrow) connected with a wire 
lead all the way at the other end of 
the PIN diodes. 

• Finally, solder a wire to each end of 
the capacitor, to connect to the 
Joule Thief later, and set the solar 
battery in the sun. 

• Be careful not to desolder 
the diode when you're 
soldering the extra leads to the 


© Make Projects 

Page 7 of 12 

Solar Joule Bracelet 

Step 5 — Build the Joule Thief. 

• Many versions of the Joule Thief 
circuit have you winding the 
transformer yourself, but since I'm 
lazy, I bought one. A common- 
mode choke, it's a surface-mount 
device (thus, very small) with a 
ferrite core. The white dot on top 
indicates pin 1, and the pins are 
numbered clockwise. 

• Start by bending the emitter pin (E) 
on the NPN transistor back 90° 
away from the flat side. 

• Then bend the collector pin (C) and 
base pin (B) so that you can solder 
C to pin 2 of the choke, and B to 
pin 4. (Note where pins 1 and 4 go 
when you turn the choke upside 

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Solar Joule Bracelet 

Step 6 — Add a resistor. 

• Position the resistor across the 
base pin, bending its leads down 
the sides of the choke, and solder 
it between pins 1 and 3. 

• Trim the pin 3 lead, but leave the 
pin 1 lead long to connect to the 
battery's positive (+) terminal. Any 
resistor from 1 K to 3K should work. 
(A larger one will be more efficient, 
but I happened to have a 1 K 
resistor on hand.) 

Step 7 — Add the axial capacitor. 

• Flip the assembly over and solder 
the axial capacitor between pin 3 
(or the resistor lead connected to 
it) and the transistor's emitter pin. 
This capacitor isn't required for the 
circuit to work, but speeds it up 
and increases efficiency. An axial 
capacitor fits better here than a 
regular disc-shaped cap would, 
making the circuit more compact. 

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Solar Joule Bracelet 

Step 8 — Add the LED jewel. 

• Bend out the LED's shorter cathode 
lead (-). 

• Solder the anode lead (+) to the 
choke's pin 2 or transistor's 
collector. Solder the cathode to the 
transistor's emitter, which should 
be conveniently poking up. 

• Trim the LED's anode but leave the 
cathode lead long. 

Step 9 — Connect the Joule Thief to the solar battery. 

• Using wires or alligator clips, wire 
the Joule Thief and the solar 
battery together, + to + and - to -. 
(The Joule Thief's positive side is 
where the resistor and LED come 
together; its negative side is where 
the transistor and axial capacitor 
meet.) The LED should glow! 

• NOTE: If you watch the 
LED's positive lead with an 
oscilloscope, you should see it 
pulsing up to the LED's forward 
voltage at 300kHz to 500kHz. 
Since that's too fast for your eyes 
to discern, it looks like a steady 

© Make Projects 

Page 10 of 12 

Solar Joule Bracelet 

Step 10 — Troubleshoot. 


• Use a voltmeter to confirm that 
your solar battery has a charge. It 
doesn't take much! Half a volt or 
more is plenty. 

• If there's no voltage from the solar 
battery, make sure the signal diode 
and capacitor are both oriented in 
the right direction. 

• NOTE: It may take many 
minutes to charge the cap 
the first time in bright sunlight, and 
maybe hours in indoor light. 

• If you have voltage but no light, 
make sure the Joule Thief is 
connected to the battery the right 
way. Check that the LED is in the 
right direction, the inductors in the 
choke are wired correctly (which is 
to say, opposite from each other), 
and that the transistor's emitter 
connects to the battery's negative 
side and its collector connects to 
the choke's pin 2. 

© Make Projects 

Page 11 of 12 

Solar Joule Bracelet 

Step 11 — Mount your Solar Joule on a bracelet. 

• The final part of the project is up to you: a crafty mounting that you provide for the 2 parts. 
That's up to your imagination! 

• We laid out the parts with the LED jewel at the head of the caterpillar of photodiodes. Then 
we sewed it to a simple cuff of neoprene salvaged from a laptop sleeve. Felt works great 
too and is easier to work with. 

• Here the LED's free lead is used as one-half of the "clasp" to connect the circuit, but you 
could strengthen or extend these connections any way you want, depending on your 

• Plan ahead and you can lay out the parts of the circuit in almost any shape you can bend 
in the solid wire! 

This project originally appeared in MAKE Volume 19 . 

Related Posts on Make: Online: 

This document was last generated on 201 2-1 0-31 09:50:46 PM. 

© Make Projects 

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